The present invention relates generally to an electrolyte measurement device and measurement procedure. More specifically, the present invention relates to an electrode configuration and method for measuring the through-thickness resistance or conductance of thin liquid, solid, or multi-phase electrolytes.
An electrolyte is an ionically conducting phase used in an electrochemical cell. Thin electrolytes are used in fuel cells, batteries, electrolyzers, dialysis cells, chloro-alkali cells, and ion selective electrode sensors, among others.
An important property of electrochemical cells and electrochemical devices is the resistance of the electrolyte. For example, in electrochemical power devices, electrolyte resistance has implications for performance, efficiency, operating requirements, durability, etc. As such, accurate measurement and characterization of the resistance or conductance of the electrolyte is of scientific and technological importance.
Those having ordinary skill in the art will recognize that resistance and conductance are governed by Ohm's law. Ohm's law states that, in an electrical circuit, the current passing through a conductor from one terminal point on the conductor to another terminal point on the conductor is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two terminal points and inversely proportional to the resistance of the conductor between the two terminal points. In mathematical terms, this is written as:
  I  =      V    R  where I is the current, V is the potential difference, and R is a constant called the resistance governed by the formula:R=(1/σ)×(L/A)=ρ×(L/A).
As shown in the equation, resistance is a function of the geometry of the system being measured. For example, in the present method, the resistance is a function of (1) the overlap area of the source electrodes (A), (2) the thickness of the electrolyte (L), and (3) the electrolyte conductivity (σ) or resistivity (ρ). Those having ordinary skill in the art will also recognize that conductivity is inversely proportional to resistivity. Stated differently:σ=1/ρ.
There are two basic approaches to resistance or conductance measurement of thin electrolytes: in-plane and through-thickness. Furthermore, the measurement is typically conducted in either a two-electrode or four-electrode configuration.
In-plane resistance measurement refers to determination of the resistance of the electrolyte in a configuration in which during the measurement the electrical current moves predominantly in the longitudinal and/or transverse direction within the thin electrolyte. This is in contrast to a through-thickness resistance measurement in which during the measurement, the electrical current moves predominantly in the short-transverse (through-thickness) orientation within the thin electrolyte.
Although in-plane resistance measurement is more easily made, it is less desired, because in-plane resistance is not the orientation of interest for most applications (i.e., in application, the direction of ion transport is through the thickness of the electrolyte and not in the plane of the electrolyte). The property of interest, therefore, is the through-thickness resistance of the electrolyte not the in-plane resistance of the electrolyte.
Furthermore, in-plane measurement may provide misleading results for electrolytes that have orientation-dependent intrinsic or effective resistivity or conductivity (i.e., anisotropic and composite material systems). Additionally, some electrolytes have non-uniform resistivity in the through-thickness orientation (e.g., a “skin” region at the surface which has different resistivity than the bulk material) and may respond differently to changes in the test conditions and material processing history. The apparent resistivity or conductivity of electrolytes within non-uniform properties obtained with in-plane measurement approaches may not be representative of the through-thickness resistivity or conductivity and consequently may provide misleading results.
Existing through-thickness measurement techniques also suffer from several drawbacks. For example, in typical two-electrode through-thickness approaches, contact resistance and lead resistance may be on the order of or exceed the electrolyte resistance. This “cell” resistance must be accounted for in order to determine the properties of the electrolyte. To determine the “cell” resistance, one must make measurements of nominally identical electrolytes of different thicknesses. This process is time-consuming, costly, and not always feasible if, for example, electrolytes of different thicknesses are unavailable. One attempt to overcome this unfeasibility was to make stacks of solid electrolytes in order to be able to determine and thus account for the cell resistances in an attempt to back-calculate the electrolyte resistance.
Additionally, charge transfer resistance and double layer capacitance at the electrode-electrolyte interface introduce artifacts that confound measurement of the electrolyte properties in two-electrode configurations. High frequency AC techniques can minimize or eliminate these effects although they typically introduce additional complexity to the measurement.
An alternative, disclosed, four-electrode through-thickness technique relies on embedding the voltage sense electrodes between layers of electrolyte. This may not be feasible under many circumstances due to, for example, lack of material availability, mechanical properties which prohibit layer stacking, such as brittle materials, unknown or unfeasible material processing characteristics required to embed the voltage sense electrodes, errors associated with voltage sense electrode placement, and test article fabrication reproducibility.
Additionally, the processing required to intimately embed the voltage sense electrodes between layers of membrane electrolytes may change the properties of the electrolyte material itself and thus produce erroneous results.
It would be desirable, therefore, to develop an method that permits simple, accurate four-electrode resistance or conductance measurement in the through-thickness direction of thin electrolytes.